This project is concerned with the physiology of the hypothalamic- pituitary-thyroid axis and the elucidation of the factors responsible for regulating secretion of thyrotropin releasing hormone (TRH), thyrotropin (TSH) and thyroid hormones. The studies in progress to realize these goals include studies on the effect of TRH on TSH secretion in humans, determination of the factors influencing serum triiodothyronine concentrations, and development and application of methods for the measurement of TRH in serum, urine and/or tissues.